Concrete or mortar resistant to spalling under fire attack

ABSTRACT

Reinforced concrete members consisting of a high-strength concrete are subject to explosive spalling. Such spalling can be prevented by the introduction of a capillary system into the concrete of such members, containing substantially linear capillaries having a diameter of at least 3 μm and a length of at least 5 mm.

BACKGROUND OF THE INVENTION

Concretes of standard DIN 1045, Eurocode 2, ACI 318-89, aredistinguished by their bulk density: lightweight concrete, normal weightconcrete and heavy weight concrete. These concretes are substantiallymanufactured from binders--in general cement of standard DIN 1164 orcomparable standards--aggregate, which meets the requirements of DIN4226 (part 1-3) and water. Concrete admixtures and additives are usuallyadditionally used. The first mentioned substances change thecharacteristics of the concrete by chemical and/or physical action, e.g.solidification, workability or setting. In contrast, additives arefinely-distributed additional substances which influence specificconcrete properties and in contrast to the concrete admixtures have tobe taken into consideration as parts by volume when calculating themixing ratios, e.g. latent hydraulic substances or pigments, which canalso be of organic origin.

Prefabricated members and buildings made of concrete--reinforcedconcrete members or prestressed concrete members--must meet a number ofrequirements in respect of bearing capacity and stability under load.The pertinant standards (inter alia DIN 1045 and DIN 4227, Eurocode 2,etc) and the building regulations of the land have to be taken intoaccount in design and manufacturing the same.

Prefabricated concrete members and concrete buildings must also meet anumber of requirements in respect of fire safety. In this connection,the building regulations of the land and, in particular, the standardsDIN 4102 (Eurocode 2, etc) are decisive.

The stability of the concrete is impaired under fire attack and theprefabricated parts exhibit failures after being exposed to fire for aspecific period of time. According to the concrete-fire protectionhandbook by K. Kordina and C. Meyer-Ottens, Betonverlag GmbH, Dusseldorf1981, pages 152 to 167, in particular the following kinds of failuresare observed in reinforced concrete members under fire: failure of thetension zone, failure due to thrust or torsional breakage, failure ofthe compression zone, failure by exceeding the admissible raise oftemperature at the non fire-exposed surface and failure due to spalling.

Destructive spalling in prefabricated concrete members of normalstrength could be counteracted by an appropriate selection of thedimensions, the cross-sectional shapes, the mechanical stressdistribution and the arrangement of the reinforcement, in connectionwith their long-term drying in the building.

Practical experience and material tests show that until now, explosionspalling of prefabricated high-strength concrete members have alwaysoccured under fire exposure. The term high-strength concretes includesthose, which, with respect to their strength, are superior to thehighest strength class B 55, embraced by the standards DIN 1045,Eurocode 2 ACI 318-89 etc, e.g. a B 85. In order to obtain the highstrength of the cement stone, high-strength concretes are made with verylow water/cement ratios generally below 0.40. Concretes are impermeableto liquid water and their diffusion of water vapor takes place veryslowly, such that the concrete--even after hundreds of year storage atambient conditions--usually contains more than 3 weight-% water. Thismeans that prefabricated members made of a high-strength concrete cannever dry out under normal ambient conditions (≦2% by weight).

Due to the prevailing high moisture content and the high diffusionresistance vis a vis water vapor, very high pressures necessarily resultinside the high-strength prefabricated concrete members under fire,which finally lead to explosive spalling, in particular when theconcrete is simultaneously subjected to high mechanical stress.

Spalling under fire has generally been observed in prefabricated memberswhose inherent moisture and impermeability exceed certain limits, forexample ≧2% in normal strength concrete. Spalling also occurs inprefabricated parts of shotcrete (according to the standard DIN 18551)or in centrifugal concrete, light-weight concrete with closed structureand shot mortar.

For this reason, very narrow limits are set to the use of these buildingmaterials, in particular of high-strength concrete, or very expensivetechnical measures, such as an outer network reinforcement forpreventing the falling off of the detached or spalled concrete core orexpensive insulations against the fast penetration of heat in the caseof fire are necessary. Also the addition of steel fibers to increase thetensile strength of the concrete did not lead to the desired success.

SUMMARY OF THE INVENTION

The object of the invention is to prevent the destructive spalling underfire in prefabricated members of dense concrete or mortar.

This is achieved by the present invention by providing prefabricatedparts of dense concrete or mortar, such as construction concreteaccording to standard DIN 1045, in particular high-strength concretes,construction lightweight concrete with closed structure, shotcrete,centrifugal concrete or shot mortar with a linear capillary system.These capillaries have preferably a circular cross section and adiameter of from 3 to 350 μm, in particular 10 to 100 μm. Lengths of upto 35 mm, in particular up to 20 mm, are usually sufficient. Thecapillaries ought to be about 0.05 to 1 vol. %, preferably 0.1 to 0.3vol. % of the concrete or mortar.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Without impairing the other desired properties of the concrete ormortar, this capillary pore system can be provided in the course oftime, e.g. during or after the hardening of the concrete, or it canfirst be produced under the fire attack. This can be effected withfibers which form the capillaries by dissolution, softening,decomposition, shrinkage or melting and which correspond with respect totheir shape, i.e. diameter and length, as well as the incorporatedamount, to the desired capillaries. The fibers can be of organic orinorganic nature. They need not consist of one and the same material,but, advantageously for the strength of the produced concrete, it canalso be a core-shell fiber, with a shell of glass or metal or carbonfiber. The core fibers are usually individual fibers. They have acoating (shell) of the same material to be removed at the latest duringfire by dissolution, softening, shrinkage, displacement or melting sothat the capillary is normally annular and also contains the carbon,metal or glass fibers centrically. Other cross-sectional shapes of thecapillaries and fibers are also possible. Also in these cases, as in thecase of capillaries with free cross section, the water in the concrete,physically bonded and partially also chemically bonded can flow out ontime when a fire occurs so that extreme water vapor pressure leading tothe familiar spalling cannot be formed. Thus, the limits for the use ofhigh-strength concrete, lightweight concrete, shotcrete shot mortar,which until now seemed to be insuperable, are abrogated by theinvention.

An inorganic fiber which dissolves in the course of time may consist,for example of glass of low resistance to alkaline solutions, e.g.alkaline solution resistance of class 3 according to standard DIN 52322,which can be attacked and dissolved by the soda alkaline liquid of thepores of the hardened cement paste with a pH value of up to 12.6. Thesame applies to organic fibers, in particular of polyester, which areprogressively saponified under these conditions. Glasses and polymers ofthe kind come into question both for the fibers as such and for theshell of the mentioned core-shell fibers.

Organic fibers which can produce the desired capillaries in the case offire consist of a material which softens, shrinks, melts or isdecomposed at temperatures of not lower than 100° C., in particular attemperatures of not lower than 150° to 300° C. Examples thereof arenatural fibers, such as wool or silk, in particular their waste orsynthetic fibers, preferably polyamides or polyolefins, such aspolyethylene or polypropylene. Also the shell of core-shell fibers canconsist of these materials.

Suitable fibers for the purposes of the invention have a diameter ofpreferably 3 to 350 μm, in particular 10 to 100 μm. The length of thefibers is advantageously 5 to 35 mm, in particular 8 to 20 mm. The fibercontent is about 0.5 to 10, preferably 1 to 5 kg/m³ concrete, or 0.05 to1, preferably 0.1 to 0.3 vol. % of the concrete.

If, beside the fibers, 2 to 6% by weight, based on the cement component,finely divided amorphous silicic acid having at least 90% by weight SiO₂or a finely dispersed precipitated active silicate of magnesium,calcium, barium or aluminum having a BET surface area of 50 to 200 m² /gand a d₅₀ %, value of below 20 μm is additionally added to the concreteor mortar mixture, the spalling during fire is reduced, surprisinglyenough, to up to zero spalling.

This result is unexpected because it is known that the dosage of activesilicic acid or active silicates of the mentioned metal ions make theconcrete more impermeable to the penetration of liquids and gases. Thereasons for the improvement of the properties of the concrete also underfire are hardly known and cannot be exactly explained.

The improvement of the fire characteristics by the finely dispersedsilicic acid and/or the silicates--due to interaction with thefibers--can be ascribed with some probability to the homogenization ofthe concrete microstructure.

Also the designed use of the described concretes or mortars withsynthetically produced micropores, or with micropores that are firstproduced upon fire exposure, which are manufactured without or withsimultaneous addition of finely dispersed silicic acids and/orsilicates, to prevent the destructive spalling in the case of fire, inparticular over the steel reinforcement, is subject matter of thisinvention.

EXAMPLE

Three concrete mixtures were manufactured according to the followingtable, on the basis of Portland cement 55 F, sand, gravel and crushedbasalt.

    ______________________________________                                        Mix proportion      I       II      III                                       ______________________________________                                        cement (PZ 55-F)                                                                              kg/m.sup.3                                                                            460     460   465                                     silica fume slurry                                                                            kg/m.sup.3                                                                            70      70    --                                      (70 kg slurry = 35 kg                                                         of silicic acid)                                                              precipitated finely                                                                           kg/m.sup.3                                                                            --      --    14.0                                    dispersed silicic acid                                                        (BAGRAT KS 300)                                                               sand 0/2 mm     kg/m.sup.3                                                                            735     735   745                                     gravel 2/8 mm   kg/m.sup.3                                                                            205     205   205                                     crushed basalt 8/16 mm                                                                        kg/m.sup.3                                                                            880     880   890                                     water           l/m.sup.3                                                                             153     153   145                                     super-plasticizer                                                                             kg/m.sup.3                                                                            27.6    27.6  28.0                                    (FM72 Sicotan)                                                                polypropylene fibers                                                                          kg/m.sup.3                                                                            4.0     --    4.0                                     (φ100 μm, 1 = 12 mm)                                                   melting interval 160-170° C.                                           water/cement + 2 × silica                                                                       0.35    0.35  0.35                                    fume                                                                          ______________________________________                                    

For the fire tests, three short columns (25×25×100 cm³ ; reinforced witheight steel bars φ 18 mm; stirrups=φ 8 cm, e=15 cm; concrete cover nomC_(B) =2.5 cm) as well as six cubes were made, moist cured for 28 daysand subsequently stored at about 65% relative moisture and 20° C. untiltesting.

The tested cubes yielded strengths between 90 N/mm² and 105 N/mm².

A total number of four three months-old columns were fire tested withthe following loads in conformity with the standard temperature curve(ISO-standard fire curve, ISO 834) DIN 4102, part 2, edition 09/77:

specimen 1, mix III, centric load with 2000 kN

specimen 2, mix I, centric load with 2000 kN

specimen 3, mix II, centric load with 2000 kN

specimen 4, mix III, excentric load (e=d/6.25) with 1200 kN.

Specimen 1, mix III, i.e. with the addition of precipitated,finely-dispersed silicic acid and the polypropylene fibers; and specimen2, consisting of mix I, with silica fume and the polypropylene fibersresisted the two hours fire exposure without showing any majordeformations, cracks or collapsing of the longitudinal reinforcingsteels. Specimen 1 did not show any spalling. Specimen 2 exhibitedbetween the 6th and 15th minute minor spalling at its smoothed topsurface which, however, were only about 5 mm in depth.

Starting from the 6th minute, serious spallings occured in sample 3, mixII, which was identical to mix I, except for the missing addition of thepolypropylene fibers, which also resulted in an exposure of stirrups andlongitudinal rods; thereupon, the test was terminated after 45 minutes.

Specimen 4, made of mix III, did not show any spalling even underexcentric load which was increased after 65 minutes to the maximumpossible load which could be achieved by the testing machine. The testhad to be interrupted due to overheating of the test frame.

The test programm was conducted with the determination of the residualbearing capacity of specimen 1, 2, and 4, around 14 days after thepertinent fire experiments. The residual capacities determined were:

specimen 1: 2740 kN (centric load)

specimen 2: 2900 kN (centric load)

specimen 4: 1720 kN (excentric load)

The short columns made of mix I and III did not exhibit any spallingalthough their concrete moisture, analogous to the column of mixII--measured by means of simultaneously manufactured and storedcubes--was between 4.2 and 4.4 mass % of the concrete. As to the columnsmade of fiber concrete, if desired with the addition of thefinely-despersed silicic acid, a classification under fire-resistantclass F 120-A was possible without reservation. Probably the columns canbe classified under fire-resistant class F 180-A. This classificationfailed only because the test had to be interrupted due to overheating ofthe test frame.

I claim:
 1. Concrete or mortar resistant to spalling under fire attack,characterized by a capillary pore system with substantially linearcapillaries having a diameter of at least 3 μm and a length of at least5 mm.
 2. A concrete or mortar according to claim 1, characterized inthat it is a high strength concrete.
 3. A concrete or mortar accordingto claim 1, characterized in that it is a shotcrete or a centrifugalconcrete.
 4. A concrete or mortar according to claim 1, characterized inthat it is a lightweight concrete with dense structure.
 5. A concrete ormortar according to claim 1, characterized in that it is shot mortar.